1. Field of the Invention
The present invention relates to image forming apparatuses, and more particularly, to an image forming apparatus that produces a predetermined image by exposing a photoreceptor using a laser beam.
2. Description of the Background Art
In an image forming apparatus such as a digital copier or a digital printer, a predetermined image is printed on a sheet of paper by exposing a uniformly-charged photoreceptor with a laser beam modulated by image data, developing the obtained electrostatic latent image with toner, and transferring this toner image onto a sheet.
The sensitivity of the photoreceptor used in the above-described image forming apparatus varies in response to temperature and sensitivity deterioration corresponding to the life time of the photoreceptor. Also, the ratio of the amount of toner transferred on a sheet to the amount of toner adhering on the photoreceptor by development (referred to as "transfer efficiency" hereinafter) varies according to humidity.
Decline in the temperature of the photoreceptor causes the surface potential of the photoreceptor to be less easily reduced to result in difficulty of adherence of the toner (i.e. concentration is reduced) even when the photoreceptor is exposed by an amount of exposure identical to that prior to temperature decline. Similarly, the surface potential is less easily reduced to prevent the toner from being readily attached as the life time of the photoreceptor comes near its end. Furthermore, there is a possibility of causing transfer inferiority depending upon the humidity in the vicinity of the transfer device.
An image forming apparatus directed to improve black line jitter, base smudge, 1-dot reproducibility, and resolution by detecting the surface potential of the photoreceptor and correcting the intensity of the laser beam according to the detected value is disclosed in Japanese Patent Laying-Open No. 2-264978.
The energy distribution of the laser beam for exposing the photoreceptor exhibits a Gaussian distribution. It is known that the shape of the distribution is always a similar figure regardless of the intensity of the laser. This means that the half-width of the laser beam does not change even when the intensity of the laser beam is varied. The half-width refers to the width of 1/2 the peak of the Gaussian distribution curve of the laser beam. The photosensitivity correction system of the above-described photoreceptor including a latent image is shown in FIG. 11. More specifically, FIG. 11 shows the relationship between the surface potential of the photoreceptor and the amount of exposure according to variation in the sensitivity characteristics. Also, the energy distribution in the subscanning direction of the laser beam and the configuration of the latent image in the subscanning direction are shown for the standard time and for the corrected time when the sensitivity characteristics of the photoreceptor changes.
Referring to FIG. 11, let the energy at the peak of the energy distribution of the laser beam for the standard time be E.sub.11, E.sub.12 (=E.sub.12 /2). The configuration of the energy distribution exhibits a Gaussian distribution. When the sensitivity characteristic is altered from C of the standard time to C', decline in the surface potential in forming a latent image becomes smaller. Therefore, the intensity of the laser beam is corrected to compensate for this effect (so that the level of the potential decline is equal to that prior to change in sensitivity). more specifically, E.sub.11, is set to be E.sub.21. Assuming that E.sub.22 =E.sub.21 /2, the beam diameter of the laser at the standard time of E.sub.12 coincides with the beam diameter (shown in d in FIG. 11) of the laser at correction of E.sub.22 since the laser distribution is similar figures regardless of the intensity of laser beam.
When a development bias V.sub.B equal to the surface potential of the photoreceptor irradiated with the exposure amount of E.sub.12 is applied to the photoreceptor under the sensitivity characteristic of C for the standard time, development at the standard time and at the correction time is carried out in the respective shaded regions in FIG. 11. Therefore, the dimension of 1 dot (dot diameter) when a latent image is represented as an image changes from d.sub.1, of the standard time to d.sub.2 of the correction time to result in a greater dot diameter. Accordingly, the area of the shaded region in FIG. 11 becomes larger to increase the amount of toner adherence. Thus, there was a problem that merely a correction of the surface potential of the photoreceptor by means of correcting the intensity of the laser beam will cause increase in the dot diameter after the correction to increase the amount of toner adherence. Thus, an image of favorable quality could not be obtained.
An image forming apparatus that detects the surface potential of the photoreceptor or the concentration of the toner image, and that corrects only the beam diameter of the laser according to the detected result to compensate for the tone characteristics is disclosed in U.S. Pat. No. 5,325,122.